1. Field of the Invention
This invention relates in general to radio frequency power divider circuits, and more particularly, this invention relates to a method an apparatus for a radio frequency power divider used in a cellular base station.
2. Background
Power divider circuits, also known as power dividers or dividers or multi-couplers, are used in radio frequency applications to split an input signal over two or more outputs so that the input signal can be processed in parallel in subsequent stages. The power level of the output signals of a power divider can be equal of weighted. For instance, a 3 decibel (dB) power divider with two outputs provides each output signal with a power level of 3 dB below the power level of the input signal.
FIG. 1A shows a basic passive configuration of a power divider arrangement using three power dividers 20, 22, 24 to provide four output signals. While the circuit is generally inexpensive, unterminated outputs (i.e., when one or more inputs is not under a load, for example 50 ohms) can cause problems such as poor input matching (resulting in power loss), large ripple in the pass-band of the circuit, and increased insertion signal loss introduced by the circuit in the terminated outputs. Further, because power dividers are relatively narrow band devices, the arrangement of FIG. 1 may result in poor isolation between the outputs, or poor out of band isolation between the outputs.
FIG. 1B shows a passive arrangement wherein attenuators 26A-26D are used at each of the four output lines of FIG. 1A. While this arrangement is also inexpensive and reduces the insertion loss when unterminated loads are present, the attenuators introduce a high amount of insertion loss into the system.
FIG. 1C shows a passive arrangement wherein isolators 28A-28D are used at each of the four output line of FIG. 1A. While this arrangement reduces insertion loss and pass band ripple, and provides effective isolation between the outputs when unterminated lines are used, this arrangement can be expensive due to the cost of the isolators.
FIG. 1D shows an active arrangement wherein active amplifiers 30A-30D and attenuators 32A-32D are used in series at each of the four output line of FIG. 1A. While this arrangement reduces insertion loss and pass band ripple when unterminated lines are used, this arrangement can be expensive due to the cost of the amplifiers, and can introduce a high noise figure which provides nonlinearity into the system. Further, a external DC power supply is needed for the amplifiers.
Another approach is to use filters at each of the output lines, which would help with out of band isolation, but would not reduce passband ripple or insertion loss.
It is with these shortcomings of the existing art in mind that the present invention was developed. What is needed is a circuit arrangement, or method, for a power divider which improves performance of the divider by improving out of band isolation, insertion loss, and passband ripple when the power divider is operating with terminated outputs or with unterminated outputs.
In light of the above, therefore, according to a broad aspect of the invention, an apparatus for an output divider which provides a default load to an output line of the power divider, when the power divider is operating with unterminated outputs. The apparatus has an input port, an output port coupled to the input port at a first node, and a voltage controlled element, such as a varactor, having a first end coupled at the first node to the input port. A second end is coupled to a resistive element, such as a resistor, with the resistive element being coupled to a signal ground. The first end of the voltage controlled element is adapted to be coupled to a reference voltage of a load, the voltage controlled element having a high impedance when the reference voltage is present and having a low impedance when the reference voltage is not present. The impedance seen from the input port is substantially the impedance of the load when the reference voltage is present, and the impedance seen from the input port is substantially the impedance of the resistive element when the reference voltage is not present. In this manner, the apparatus, when coupled to an output line of a power divider, provides a default load, as seen from the output line of the power divider, when the output line of the power divider is unterminated. When the output line of the power divider is terminated with a load, the impedance as seen from the output line of the power divider is substantially the impedance of the load. In one implementation, even when one or more output lines of a power divider are unterminated, the apparatus of the present invention is adapted to provide a default load to each output line of the power divider, thereby improving performance when compared with conventional power dividers having unterminated output lines.
The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.